Biaxially oriented polyester containers produced by shaping preforms made of polyester resins, such as polyethylene terephthalate (PET), into the form of a wide-mouthed bottle or a usual bottle with biaxial orientation blow molding are superior in transparency, impact resistance and gas barrier property, and are widely employed in various fields including foods, seasonings and beverages in applications requiring resistance against heat, heat and pressure, and pressure.
In applications requiring resistance against heat, for example, such a biaxially oriented polyester container is produced through the steps of heating a preform, of which mouth part has been subjected to a crystallizing process, to temperatures not lower than the glass transition point (Tg), performing biaxial orientation blow molding of the heated preform using a biaxial orientation blowing mold, and heat-setting a barrel part of the container at temperatures (100-150° C.) not lower than the crystallization temperature so as to remove residual stresses generated during the biaxial orientation blowing and to provide heat resistance.
With such single-stage biaxial orientation blow molding, however, because a polyester container is formed by performing the biaxial orientation blow molding in a state in which a bottom part of a preform 10 is restrained by a stretch rod 14 and a press rod 15 as shown in FIG. 11, the bottom part of the polyester container is not sufficiently elongated and a non-elongated portion remains in or near a bottom center area of the polyester container.
Accordingly, if the bottom part is heat-set at high temperatures to be adapted for applications requiring resistance against either heat or heat and pressure, the bottom part is whitened due to thermal crystallization and a commodity value of the final product deteriorates. For that reason, the bottom part has been heat-set in the range of about 75-100° C., and an application range of the thus-produced polyester container has been limited correspondingly. Further, the non-elongated portion has a thick wall and hence impedes a reduction of weight of the polyester container.
On the other hand, another known biaxial orientation blow molding method comprises the steps of, as shown in FIG. 12, heating a preform 10, of which mouth part has been subjected to a crystallizing process, to temperatures not lower than the glass transition point (Tg), performing primary orientation blow molding of the heated preform with a primary mold to obtain a primary molded product 20 larger than a final molded product, heat-shrinking the primary molded product 20 into a secondary molded product 21, and performing secondary orientation blow molding of the secondary molded product 21 with a secondary mold, thereby obtaining a polyester container 31. (See, e.g., Japanese Patent Laid-Open No. 9-216275).
The above known method enables the bottom part of the polyester container to be elongated at a higher rate and to have a thinner wall. As in the single-stage biaxial orientation blow molding, however, the biaxial orientation blow molding to obtain the primary molded product with the primary mold is performed in a state in which a bottom part of the preform 10 is restrained by a stretch rod 14 and a press rod 15 as shown in FIG. 11. Therefore, the primary molded product is not sufficiently elongated in or near a bottom center area thereof. For that reason, even when the polyester container is formed by the double-stage orientation blow molding, a non-elongated portion remains in or near the bottom center area of the container. Hence, similar problems to those mentioned above are left unsolved.
Further, when the polyester container is formed by performing the biaxial orientation blow molding in the state in which the bottom part of the preform 10 is restrained by the stretch rod 14 and the press rod 15, a surrounding area of the restrained portion is highly uniaxially oriented and a plastic container made of polyester and the like tends to crack in the direction of the uniaxial orientation, thus causing problems in points of drop strength and ESC (Environmental Stress Cracking) resistance. An attempt of overcoming those problems with an improvement of the mold shape requires a bottom part of the mold to be formed into a complicated shape, takes a longer time for mold design, and hence increases the production cost.
Accordingly, it is an object of the present invention to solve the above-mentioned problems in the art, and to provide a biaxially oriented polyester container with a uniformly and sufficiently elongated and thin-walled bottom part having excellent drop strength, improved ESC resistance, and reduced weight, and to provide a method of manufacturing the container.